1. Field of the Invention
The invention pertains to fuel cells with a polymer electrolyte, also known as the proton-exchange-membrane type. More particularly, the invention pertains to a polymer electrolyte fuel cell which includes an injection molded external housing.
2. Description of the Prior Art
In the prior art, fuel cells have been categorized into five types according to the nature of the electrolyte employed in the cell, namely, alkaline, phosphoric acid, molten carbonate, solid oxide and polymer electrolyte. The present invention pertains to polymer electrolyte fuel cells, also known as the proton-exchange-membrane (PEM) type. In the PEM cell, the electrolyte is comprised of a thin membrane made of a polymer similar to polytetrafluoroethylene (PTFE or Teflon.RTM.) with sulfonic acid groups included in the polymer molecular structure. The sulfonic acid groups are acid ions which are the active electrolyte. The membrane has the dual attributes of readily conducting hydrogen nuclei (H.sup.+ ions or protons) from one face through the thickness of the membrane to the opposite face while effectively blocking the flow of diatomic hydrogen molecules through the membrane. Thus, if a chemical reaction can be made to occur on one face whereby diatomic hydrogen gas (H.sub.2) can be reduced to protons (2H.sup.+) and electrons, the protons flow through the membrane, while the electrons can be passed through an external electrical conductor. Then, if a second reaction which oxidizes the protons can be made to occur on the opposite face, a continuous flow of protons across the membrane, and of electrons through the external conductor occurs thereby producing electrical current. In the PEM fuel cell, the two reactions are:
H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- at the anode side of the cell and PA1 O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O at the cathode side of the cell PA1 1. Hydrogen gas must be distributed uniformly over the active area of the anode side of the membrane. PA1 2. Oxygen or air must be distributed uniformly over the cathode side of the membrane. PA1 3. The membrane must be kept moist to a controlled degree. PA1 4. A catalyst must be uniformly dispersed over the active area on both sides of the membrane in such a manner that each catalyst-particle site is concurrently accessible to the reactant gas, the polymer electrolyte material, and to a third material which forms an electrically conductive path. PA1 5. A means must be provided to collect the electron flow (electrical current) over the entire area of the membrane, and ensure an uninterrupted electrically conductive flow path from the catalyzed surfaces of the membrane to these current-collector devices. PA1 6. The channels or chambers containing the reactant gas must be sealed from one another and from the ambient atmosphere, to prevent both wasteful loss of the gases and potentially dangerous mixing of the reactants inside the cell. PA1 1. It allows reactant gas to diffuse under the graphite plates between the grooves, enabling the inter-groove areas of the membrane to become active and generate current. PA1 2. It forms the electrically conducting path for current generated in the groove regions of the cell to flow laterally to areas where the contacting portions of the graphite plates can conduct it perpendicularly through the stack. PA1 3. It acts as an electrically conducting material between the current-producing, catalyzed sites on the membrane and the graphite plates, resiliently conforming to surface irregularities and improving the electrical contact between these members. PA1 4. It bridges across the grooves in the graphite plates, providing some structure support for the membrane. The membrane is a thin, somewhat fragile material, ranging from 0.0008 to 0.007 inches in thickness.
In order that such a membrane performs as a fuel cell and produces electric current in an effective manner, several functional requirements must be satisfied, to wit:
Single cells produce voltage in the range of 0.4 to 0.8 volts. Practical applications require that multiple cells be assembled to be in series electrically, enabling the delivery of current at voltages from 6 to 120 volts. Such assemblies are referred to as "stacks" and the cells are sequentially physically stacked and clamped together.
FIGS. 1A and 1B show a representative construction of a PEM fuel-cell stack 200 of the prior art which is comprised generally of successive planar elements as described hereinbelow. The exterior of the fuel-cell stack 200 is formed from upper steel clamping plate 201 and lower steel clamping plate 230, with electrically insulating layers 202, 221, respectively, inwardly adjacent therefrom. Similarly, copper current collection plates 203, 222 are inwardly adjacent from the electrically insulating layers 202, 221, respectively, and graphite anode plate 204 and graphite cathode plate 223 are inwardly adjacent from copper current collection plates 203, 222, respectively. The graphite anode plate 204 includes serpentine machined hydrogen distribution grooves 208 (shown in cross section) which are oriented inwardly to be exposed to carbon-cloth gas diffusion anode layer 205 to distribute hydrogen gas over the active area of the top cell formed from carbon-cloth gas diffusion anode and cathode layers 205, 205' surrounding polymer membrane 206. Serpentine machined hydrogen distribution grooves 208 receive hydrogen via horizontal spur passages 211 from gas supply manifold passage 210 which is an aperture extending downwardly through the various planar elements.
Similarly, graphite cathode plate 223 includes serpentine machined air distribution grooves 229 (shown in cross section) which are oriented inwardly to be exposed to cathode carbon-cloth gas diffusion layer 224. Serpentine machined air distribution grooves 229 receive air from an air supply manifold passage and associated passages (not shown) similar in construction to gas supply manifold passage 210.
Polymer membranes 206, 225 and carbon-cloth gas diffusion cathode and anode layers 205', 226 are successively inwardly adjacent from carbon-cloth gas diffusion layers 205, 224, respectively. Sealing gaskets 214 seal the periphery of the various carbon-cloth gas diffusion layers.
The interior of the fuel cell stack 200 includes alternating layers of graphite bipolar plates 207 (which include serpentine machined air distribution grooves 209 similar to elements 229 and serpentine machined gas distribution grooves 213 in fluid communication with spur passage 212, similar to elements 208, 211, respectively) and intermediate cells formed from polymer membrane 220 (similar to polymer membrane 206) surrounded by carbon-cloth diffusion anode and cathode layers 219, 219' (similar to 205, 205').
Carbon-cloth gas diffusion layers 205, 205', 219, 219', 224, 226 provide gas diffusion layers between the grooved surfaces of plates 204, 207, 223 and the polymer membranes 206, 220, 225 and are composed of a cloth woven of carbon fibers, with a slurry of lampblack and a small portion of polytetrafluoroethylene (PTFE or Teflon.RTM.) impressed and sintered into the interstices of the fabric. Each polymer membrane thereby resides between cushioning "blankets" of carbon cloth infused with carbon and PTFE particles, in turn clamped between grooved, graphite plates. The catalyst, usually platinum, is applied as a slurry or paste of platinum-black and lampblack in a dilute solution of the polymer of which the membrane is comprised. Two different approaches have evolved with regard to the catalyst, the gas-diffusion cloth and the membrane. In the first approach, the catalyst is included in the slurry applied to the surface of the gas-diffusion cloth and the membrane. The two cloth layers are then placed next to the membrane, one on each side, and this three-layer sandwich is hot-pressed together. The polymer component of the slurry bonds to the membrane, uniting the three layers to form an integral structure called the membrane-electrode assembly (MEA). The second approach employs an ink comprised of minute particles of platinum supported on lampblack particles, suspended in a solution of the polymer material. The ink-slurry is applied to both surfaces of the membrane, which is then hot-pressed to bond the ink onto the membrane. The polymer component of the ink intimately bonds to the polymer material of the membrane. The ink-coated membrane is referred to as the MEA in this approach. A layer of un-catalyzed gas diffusion cloth is then placed adjacent to each side of the membrane when the cell is assembled. In this approach, the catalyzed membranes must be accurately located and assembled into the stack structure as an unsupported, thin material.
The gas diffusion layer serves four purposes:
Near both the top and the bottom of the stack are current-collector plates 203, 222, typically made of copper, which collect the current produced over the entire area of the cells. Protruding tabs (not shown) on these plates are connection points for external electrical conductors. At the top and bottom of the stack are rigid clamping plates 201, 230, usually made of steel or aluminum, with apertures 217, 218 through which tie bolts 215 are passed and secured with nuts 216. These members are used to apply a compressive pressure on the order of 120 psi to clamp the stack components together. This is necessary to obtain good electrical connection between all the components over the entire active area of the cells. These clamping plates 201, 230 and inter-connecting tie rod(s) 215 must be electrically insulated from the current-collector plates 203, 222, or the stack 200 will be short-circuited through the tie rod(s) 215. Therefore, electrically insulating layers 202, 221 are interposed between the current-collector plates 203, 222 and the clamping plates 201, 230.
However, the prior art configuration shown in FIGS. 1A and 1B is less than desirable due to its high weight, cost and volume per kilowatt due primarily to the machined graphite plates, the clamping plates and tie bolts. Additionally, sealing between the stacked components has been difficult, both around the perimeter of the cells, and around manifold passages internal to the stack. The polymer membrane has been difficult to handle and position during assembly, as it is merely a thin, unsupported material having rigidity comparable to common plastic wrap.